A cased wellbore typically possesses an annular space between the casing and the formation wall that is permanently sealed by being filled with cement. This layer of cement is typically referred to as a “cement sheath.” A properly formed cement sheath should fill all or nearly all of the annular space and should bond tightly to the casing and the formation.
Both sonic and ultrasonic waveforms have been used to evaluate the quality of this cement bond from a logging tool inside the casing. The logging tool, which may have one or more sonic or ultrasonic receivers and one or more sonic or ultrasonic transmitters, is lowered into a wellbore and measurements are taken at various depths. Sonic or ultrasonic waves are transmitted from the logging tool in the wellbore, and reflected waves from the casing, cement, and formation are received, recorded, processed, and interpreted to evaluate the presence and quality of the cement sheath and bond in the annular space between the casing and the formation wall.
Processing received waveforms to produce cement bond logs, however, is a highly subjective skill that is prone to substantial error and variation among different interpreters. In addition, the cement bond logs that are produced only provide bonding information pertaining to limited portions of the cement sheath—that is, they provide an incomplete and often misleading picture of the true cement bonding status of the sheath as a whole. Further, it is challenging to produce accurate cement bond logs in wellbores with multiple concentric casing strings where the cement sheath being evaluated is disposed outside of the outer casing string, since the concentric casing strings may interfere with the sonic or ultrasonic waves transmitted to the cement sheath and reflected back from the cement sheath. Thus, methods and systems for generating accurate and consistently reproducible cement bond logs that visualize most or all of the cement sheath, including in multi-string environments, are desired.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.